I noticed even though some of your pages are marked with .xhtml extensions, they're being served up from the server as HTML. This means it's being fed through the browser's HTML decoder rather than its XML/XHTML decoder. The correct HTTP Content-Type header should be application/xhtml+xml and must be sent by the server HTTP response, not just as text inside the document itself. True XHTML will NOT work in IE6 through IE8, that's how you'll know. In all honesty it should be normal HTML with the HTML5 doctype, as true XHTML will cause a mess with legacy javascript libraries and older browsers.
"true XHTML" means XHTML parsed as an XML document.
See http://www.w3.org/TR/xhtml-media-types/
TL;DR XHTML pages being served up as HTML, and in all honesty it really should simply be HTML served up as HTML in the first place.